The invention relates to a method for setting and for fine-tuning the breakover voltage on thyristors that have already been fabricated.
A thyristor has a semiconductor body which has, in succession, an n-doped cathode-side emitter, a p-doped cathode-side base, a n-doped anode-side base and a p-doped anode-side emitter and whose cathode-side emitter is provided with an electrode serving as a cathode and whose anode-side emitter is provided with an electrode serving as an anode. If an electrical voltage is applied to the anode by the positive pole and to the cathode by the negative pole, the thyristor is in the blocking region (blocking state) for low voltage magnitudes. If the voltage is increased, a current flow in the forward direction is turned on when the (maximum) forward blocking voltage is reached, the so-called breakover voltage, also referred to as trigger voltage. The thyristor is said to trigger. If the voltage is polarized oppositely between anode and cathode, the thyristor is in the blocking region, which is limited by the reverse blocking voltage at which the thyristor breaks down, with the result that a current flows in the reverse direction.
The thyristor can also be triggered in the presence of a voltage in the blocking region. This triggering can be effected by a control current which is applied between the cathode-side base and the cathode-side emitter, and this can be done, for example, via a so-called gate electrode in a region of the semiconductor body provided for this purpose. In the absence of a control current, the triggering can also be effected as so-called breakover triggering by increasing the voltage to the value of the breakover voltage, in addition through the use of a sufficiently steep voltage rise in the blocking direction or by the radiation of light into a region of the semiconductor body formed specially for this purpose, predominantly the cathode-side base.
The breakover voltage is limited by different effects, depending on the configuration of the thyristor. A current flow in the forward direction can be turned on by the electric field at the blocking cathode-side pn junction becoming so large that charge carrier multiplication (avalanche breakdown, avalanche breakover) occurs, or by the space charge zone extending virtually over the entire thickness of the anode-side base and approaching the anode-side emitter to an extent such that the current gain factor of the anode-side transistor formed from the cathode-side base, anode-side base and anodeside emitter becomes large enough to switch the thyristor on (so-called punch-through effect). The doping profiles of the base regions essentially determine which of the two effects leads to a breakover or breakdown.
Published, Non-Prosecuted German Patent Application No. DE 196 50 762 A1 describes a thyristor in which a zone swept over by the space charge zone (depletion zone) in a region provided for breakover triggering is irradiated with xcex1 particles or protons in order to modify the profile of the charge carrier lifetime in such a way that the voltage required for breakover triggering is limited and becomes largely temperature-independent in the range of the operating temperatures. In this thyristor, the breakover triggering takes place in a central BOD structure (breakover diode), which is provided for limiting the breakdown voltage.
U.S. Pat. No. 4,987,087 describes a production method for thyristors, in which the otherwise completed thyristor is irradiated with protons whose penetration depth is set to the cathode-side half of the anode-side base. Defects are produced there in this way, which defects act as donors and locally increase the charge carrier concentration in order to lower the breakover voltage. Although the avalanche breakdown voltage is lowered as a result, on the other hand the current gain factor of the anode-side transistor is lowered due to the reduction of the extent of the space charge zone, with the result that controlled lowering of the breakdown voltage is very difficult.
It is accordingly an object of the invention to provide a method for setting a breakover voltage of a thyristor which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which can be used to set the breakover voltage of completed thyristors.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for setting a breakover voltage of a thyristor, the method includes the steps of:
providing a thyristor with a semiconductor body having, in succession, an n-doped cathode-side emitter, a p-doped cathode-side base, an n-doped anode-side base and a p-doped anode-side emitter, and the semiconductor body having a given region in which a current flow is triggered if a forward-biased electrical voltage between the anode-side emitter and the cathode-side emitter reaches a breakover voltage; and
radiating ions into the given region provided for triggering the current flow and producing, with the ions, defects in a cathode-side half of the anode-side base such that the defects counteract dopant atoms present in the cathode-side half of the anode-side base.
In other words, a method for setting a breakover voltage of a thyristor, which has a semiconductor body having, in succession, an n-doped cathode-side emitter, a p-doped cathode-side base, an n-doped anode-side base and a p-doped anode-side emitter, and in which the semiconductor body contains a region in which a current flow is triggered if a forward-biased electrical voltage between the anode-side and the cathode-side emitter reaches the value of the breakover voltage, wherein ions are radiated into the region provided for triggering and wherein the ions generate defects in a cathode-side half of an anode-side base, and wherein the defects counteract the dopant atoms.
In the method according to the invention, the effective doping profile of the completed thyristor is altered by irradiation with a suitable ion type in a region provided for triggering, e.g. in a BOD (breakover diode) structure, in such a way that the breakover voltage is set, at least within unavoidable tolerances, to a predetermined value. The ion type is chosen such that defects are produced as a result of the radiating-in process in the semiconductor material, which defects counteract the dopant atoms. What are most effective in this case are alterations in the more weakly doped zone, generally the anode-side base and there preferably in the vicinity of the pn junction between the anode-side and cathode-side bases. In particular, helium ions are suitable for the modification of the n-doped base, the helium ions producing acceptor-like states which lower the effective n-type doping in a region in which the helium ions are highly decelerated. The region in which the helium ions are highly decelerated is generally situated just before the maximum penetration depth which can be determined at least approxiamtely.
Accordingly, a preferred mode of the invention includes radiating ions into the anode-side base such that the ions produce defects acting as acceptors in the anode-side base. Also, a preferred mode includes radiating the ions into the anode-side base such that the ions produce defects acting as acceptors in a zone of the anode-side base adjoining the cathode-side base.
According to another mode of the invention, the ions are radiated into a portion of the thyristor including a BOD structure.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for setting the breakover voltage of a thyristor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.